Heated chemical conversion filter assembly for aerosol analysis

ABSTRACT

IMPROVED HEATER MEANS-PREFILTER ASSEMBLY AND METHOD FOR UTILIZATION IN NERVE GAS DETECTION DEVICES, WHEREBY AIR CONTAINING AEROSOL IS SAMPLED, THE AIR SAMPLE IS PASSED OVER A PREFILTER ELEMENT TO WET THE PREFILTER ELEMENT SURFACES WITH AEROSOL, AND THE SURFACE OF THE PREFILTER ELEMENT IS HEATED BY AN ISOLATED HEATER MENAS FROM THE NONACTIVE SIDE TO CONVERT THE AEROSOL PARTICLES ON THE PREFILTER ACTIVE SURFACE TO VAPOR ANALOG TO PASS TO A VAPOR DETECTING MEANS.

H. R. CARLON March .2, 1971 HEATED CHEMICAL-CONVERSION FILTER ASSEMBLYFOR AEROSOL ANALYSIS Filed June 15, 1967 .all

PRIOR ART INVENTOR. Hug/1 R. Car/0n ATTORNE Y5 United States Patent 13,566,672 HEATED CHEMICAL CONVERSION FILTER ASSEMBLY FOR AEROSOLANALYSIS Hugh R. Carlon, Edgewood, Md., assiguor to the United States ofAmerica as represented by the Secretary of the Army Filed June 15, 1967,Ser. No. 646,801 Int. Cl. G01n 31/10 US. CI. 73-23 2 Claims ABSTRACT OFTHE DISCLOSURE Improved heater means-prefilter assembly and method forutilization in nerve gas detection devices, whereby air containingaerosol is sampled, the air sample is passed over a prefilter element towet the prefilter element surfaces with aerosol, and the surface of theprefilter element is heated by an isolated heater means from thenonactive side to convert the aerosol particles on the prefilter activesurface to vapor analog to pass to a vapor detecting means.

My invention relates to an improved nerve gas detection device wherein aheater means is arranged to provide for positive detection of a nervegas.

Nerve gas devices must respond to chemical agents, which are normallyvapors as well as those which have very low vapor pressures. Suchnonvolatile agents normally are dispersed as aerosols having particlesizes generally lying between 1 and 50 microns. Detectors presently inuse will not respond chemically to toxic agents normally encountered asan aerosol, and such detectors have included in them a chemicalconversion prefilter which is designed to convert aerosol droplets drawninto the prefilter to the normal gaseous agent, or vapor analog, that isreadily detected. The conversion reaction of aerosol to vapor analogwill proceed well only above or near room temperature. Therefore, incold weather operation, the aerosol laden air must be heated in order toassure conversion to the vapor analog of the aerosol droplets stopped bythe filter. The need for a heating system brings about the problem forrequiring much power to operate the detector; the room temperature powerrequirement is at least doubled under arctic conditions. In addition, anappreciable pressure drop across the prefilter in prior art devicescreated a pumping resistance, and a crack in the prefilter, a commonoccurrence due to the fragility of the item, prevented aerosol detectioninasmuch as the air stream took the path of least resistance through thecrack. In view of these problems, the prior art detectors required largearea filters to reduce the pressure drop, critical gasketing, and adevice which was diificult to service. Initial attempts in the prior artdevices to solve the above problems sought merely to incorporate aheater in advance, or at the inlet side, of the prefilter. However, thisarrangement proved unsatisfactory due to the conductive losses to thesurrounding supports which were equal in power dissipation to the usefulheating of the air stream, This was due, of course, to the fact thatair-to-solid heat transfer is much less eflicient than solid-to-solidheat transfer, especially at low air flow rates. The prior art devicerequired heating the entire air stream in order to heat the aerosoldroplets contained in it, and was wasteful of power. Conductive losses,although minimized, were as significant as the convective heatingrequired.

It is a specific object of my invention to provide a nerve gas detectorprefilter assembly which provides positive detection at all temperaturesat significant power saving.

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It is a further object of my invention to provide a detector which doesnot require large area filters, does not require gasketing, eliminatespressure drop across the pre filter, is easily serviced, and heatssubstantially only the prefilter and thus the aerosol impacted thereonand not the whole air stream. I

Other objects of my invention will be obvious or will appear from thedisclosure hereinafter set forth.

According to my invention, air laden with aerosol particles enters at 1in FIG. 2. The air stream impinges upon conversion prefilter 2 in FIG. 2with a velocity determined by the geometry and flow rate of the assemblyand system. The prefilter 2 is heated by a heating element 4 in contactwith the inactive surface of the prefilter which brings about eflicientheating due to direct conduction by means of solid-to-solid contact. Theheater 4 is mounted in a thermally insulating tube 3 which is readilyremovable for replacement and cleaning of the prefilter, and the heaterelement is isolated from the air stream by insulation means; tube 3serving as a bafile means to assist in the elimination of pressure dropacross prefilter 2. The prefilter geometry is designed for a givenaerosol particle size based on flow rates anticipated to be encountered.Particle sizes of aerosol larger than the design diameter will impingeon the prefilter, wet the surface, be chemically reacted to form vapor,and pass upward through the annulus 6 in FIG. 2. Thus, the dropletswetting the prefilter surface are heated to prefilter temperature, theaerosol is thus converted to vapor analog, and the air stream continuesonward without carrying away valuable power in the form of heat. This,of course, is due to the prefilter alone being heated and not the airstream due to the insulation means. The detection system of my inventionoperates with a high degree of efficiency because only throughconversion to a vapor can the toxic agent be carried upward through theannulus and on to detection by the air stream. The only cooling effect,power waste, other than the small conductive loss to tube 3 in FIG. 2,is that due to the passage of low velocity air over the filter surfacewhich is small due to the inefficiency of the gas-to-solid type oftransfer. Illustrative of the power savings for arctic conditions; theprior art devices would require 5 to 8 watts to preheat the systemproperly, whereas my invention only requires 1 watt under the sameconditions.

FIG. 1 is a cross sectional view of the prior art detector.

FIG. 2 is a cross sectional viewof my improved detector. I will nowdescribe the figures in detail:

FIG. 1 shows a housing 10 containing a prefilter element 2, gasket 9,and heater means 8. As can be seen, the air stream is heated and notmerely the prefilter element 2. Any crack in the prefilter allows theairstream and aerosol to pass straight through.

FIG. 2 shows a detector housing 7 enclosing prefilter element 2 havingan active surface to chemically convert aerosol particles to vaporanalog and a non-active surface, heater element 4, and support element3. Heater element 4 is mounted to contact the non-active side ofprefilter element 2 and the prefilter-heater elements are mounted onsupport 3. An annulus space 6 is provided as a result of theconfiguration of support 3 and housing 7, and permits the passage of airladen with vapor analog to exhaust to space 5 for transmission to thedetection element 11.

It is obvious that other modifications may be made, and I desire that myinvention be limited only by the scope of the appended claims.

I claim:

1. A gas detection device for detecting the presence of toxic gases inan air stream containing aerosol particles including an open housingwherein the improvement in combination therewith comprises a chemicallyactive conversion element to convert the aerosol particles to a vaporanalog within said open housing, said element having a chemically activesurface and a chemically inactive surface opposite to the activesurface; a heater means contacting the chemically inactive surface ofsaid element, said heater means being in a chamber means and isolatedfrom the air stream by insulation means; insulation means contactingeach end of the conversion element to form a chamber to isolate theheater means from the air stream; a bafile means in juxtaposition to theinsulation means whereby a pressure level is sustained across thechemically active conversion element; and a conduit means to transportthe vapor analog to a detection means.

2. A method of detecting gases comprising the steps of providing a meansto sample air containing aerosol particles from an air stream, isolatinga heater means for a conversion element from the air stream to provideinsulation means, passing the sample of air over a chemically activesurface of the conversion element, Wetting the conversion elementchemically active surface with the aerosol particles Within the airsample, heating the chemically active surface of the conversion elementfrom a chemically non-active side of the conversion element to convertthe aerosol particles on the chemically active surface of the conversionelement to a vapor analog, converting the aerosol particles by chemicalreaction on the chemically active surface of the conversion element tothe vapor analog, sustaining a pressure level across the chemicallyactive surface of the conversion element through restricting flow of theair stream by a baffie means, passing the air stream containing thevapor analog to a gas detecting means, and detecting the gas.

References Cited UNITED STATES PATENTS 2,033,489 3/1936 Silten 219-2752,208,130 7/1940 J-aroll 219-271 2,599,485 6/1952 Robinson 219-2713,245,250 4/1966 Parks, Jr. 7323 3,300,282 1/1967 Risk, et a1. 73231,934,433 11/1933 Lacy 23288 3,431,083 3/1969 Beogstraud 23288 RICHARDC. QUEISSER, Primary Examiner E. I. KOCH, Assistant Examiner

